The present invention relates to shaft seal structure for a vacuum pump that drives a gas transferring assembly due to rotation of a rotary shaft, generates vacuum action by transferring gas due to motion of the gas transferring assembly.
Japanese Unexamined Patent Publication No. 6-101674 discloses a vacuum pump that drives a gas transferring assembly due to rotation of a rotary shaft, generates vacuum action by transferring gas due to motion of the gas transferring assembly. This kind of vacuum pump has a plurality of rotary shafts that support each rotor or gas transferring assembly, and the rotary shafts are synchronously driven through a gear mechanism. The gear mechanism is lubricated by lubricant prepared in an oil bath in a gear case. The lubricant is also used for lubricating bearings which rotatably support the rotary shafts.
To prevent the lubricant in the oil bath from leaking into a pump chamber which accommodates the rotors in a housing, lip seals are disposed at the surfaces of the rotary shafts between the bearings and the housing.
An unwanted effect of the lip seal is that the large pressure difference between the two surfaces of the lip seal causes the lubricant in the gear case to leak into the pump chamber, with a consequent of deterioration of the durability of the lip seal and shortened lifetime of the lip seal.
A screw type vacuum pump disclosed in Japanese Unexamined Patent Publication No. 6-81788 is provided with an annular recess formed on a bottom end surface of a screw rotor, and a discharge port of the vacuum pump opens so as to wrap over a part of the recess as seen in an axial direction of a rotary shaft. Pressure in the discharge port is applied to a back surface of a lip seal via the recess. Thereby, the pressure difference between the two surfaces of the lip seal can be reduced.
However, in a roots pump, cocoon-shaped rotors are engaged with each other so that forming annular recesses at the rotors so as to communicate with a discharge port is difficult since the location of the discharge port is restricted within a limited space.
The present invention addresses the above-mentioned problems traceable to a pressure difference applied to surfaces of a lip seal by reducing the pressure difference. Accordingly, it is an object of the present invention to improve sealing performance and lengthen the lifetime of the lip seal by reducing pressure difference between the two surfaces of the lip seal.
According to the present invention, a vacuum pump has a housing accommodating a rotary shaft and a gas transferring assembly driven by the rotary shaft. The housing has an exhaust passage for exhausting gas outside the housing. The gas transferring assembly creates a vacuum. A lip seal for shaft seal is disposed between a pump chamber communicating with the exhaust passage and a region in which oil exists so as to slide relative to a circumferential surface of the rotary shaft. The lip seal has a back pressure surface in a back pressure region facing the pump chamber and a pressure surface facing the region in which the oil exists. A communicating passage in the housing intercommunicates the back pressure region and the exhaust passage for applying at least substantially the pressure in the exhaust passage to the back pressure surface. Thereby, at least substantially the pressure in the exhaust passage is applied to the back pressure surface of the lip seal. Accordingly, the difference between the pressures applied to the pressure surface and the back pressure surface is reduced.
Either pressure in a highest pressure region in the pump chamber communicating with the exhaust passage or the pressure in the exhaust passage is applied to the back pressure surface of the lip seal via the communicating passage. This can reduce the difference between the pressures applied to the two surfaces of the lip seal, as compared with structure without the communicating passage.
The present invention has such a feature that a region to which substantially the same pressure as the exhaust passage is applied is the highest pressure region. The pressure in the highest pressure region is applied to the back pressure surface of the lip seal via the communicating passage.
The pressure in the highest pressure region is applied to the back pressure surface of the lip seal via the communicating passage. Such structure for applying the pressure in the highest pressure region to the back pressure surface via the communicating passage can reduce the pressure difference between the pressures applied to the two surfaces of the lip seal, as compared with structure without the communicating passage.
The present invention has the following feature that the housing forming the communicating passage includes a dividing wall. The dividing wall divides the region in which the oil exists and the pump chamber communicating with the exhaust passage. The rotary shaft extends through a bore in the dividing wall from the pump chamber into the region in which the oil exists.
The communicating passage is formed in the dividing wall. The pressure in the highest pressure region is applied to the back pressure surface of the lip seal via the communicating passage.
The present invention has such a feature that the dividing wall provides a wall surface defining the pump chamber. The communicating passage is a recessed channel in the wall surface. The channel extends to the dividing wall bore.
The pressure in the highest pressure region or the pressure in the exhaust passage is applied to the back pressure surface of the lip seal via the recess and a gap between the circumferential surface of the rotary shaft and the shaft hole.
The present invention further includes a first extending portion formed on the circumferential surface of the rotary shaft so as to reduce a gap between the circumferential surface of the rotary shaft and the shaft hole. The recess reaches the shaft hole so as to pass by a part of the extending portion.
The present invention further includes a second extending portion formed on the rear end surface of the rotor so as to reduce a gap between the rear end surface of the rotor and the dividing wall. The recess reaches the shaft hole so as to pass by a part of the second extending portion.
The first and second extending portions are efficient in applying the pressure in the highest pressure region or the pressure in the exhaust passage to the back pressure surface of the lip seal.
The present invention has the following feature that the region in which the oil exists is a region accommodating a bearing for rotatably supporting the rotary shaft.
The oil lubricating the bearing also lubricates the lip seal.
The present invention further includes a feature as follows. The vacuum pump is a roots pump. The gas transferring mechanism has a plurality of generally parallel rotary shafts. Each of the rotary shaft carries a rotor with adjacent rotors. The adjacent rotors are engaged with each other. A set of the engaged rotors is accommodated in either a plurality of the pump chambers or the single pump chamber.
Such vacuum pump as a roots pump is appropriate for applying the present invention.
The present invention has such a feature that a plurality of the rotary shafts is synchronously driven through a gear mechanism. The region in which the oil exists includes a region accommodating the gear mechanism.
The oil lubricating the gear mechanism also lubricates the lip seal.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings.